This study investigates the influence of pre-crack conditions (introduced under Mode I and Mode II loading prior to fracture testing) and specimen compliance on the Mode II fracture characterization (G_IIC) of adhesively bonded composite joints. Calibrated End-Loaded Split (CELS) and 3-Point-Bending ENF tests were performed using structural AF163-2 K adhesive. Various data reduction schemes were employed to account for pre-crack morphology and compliance in the development of the R-curve. The data reduction schemes showed significant scatter, ranging from 8.07 ± 0.17 to 17.3 ± 1.19 N/mm, depending on the pre-cracking conditions and compliance effect. Mode I pre-cracked specimens consistently exhibit higher G_IIC values compared to Mode II pre-cracked specimens, a difference governed by the morphology and extent of the fracture process zone (FPZ). Mode I pre-cracking forms a localized FPZ that subsequently transitions into a shear-dominated FPZ for G_IIC evaluation during the subsequent Mode II fracture test. In contrast, Mode II pre-cracked specimens contain an already-developed shear FPZ that is broader and more diffuse, resulting in lower strain-energy release rates and lower G_IIC values. High compliance effects cause significant bending, additionally introducing high derogatory energy deformation from the test fixtures, obscuring the actual crack tip. The apparent crack length methods demonstrated reliable estimates of fracture energy and R-curve behavior by accounting for the effects of large FPZ, thereby capturing both crack-tip and distributed dissipation mechanisms. The experimental findings correlate with computational results, displaying stable cohesive disbond growth in the adhesive layer. This study indicates that pre-cracking and compliance effects significantly influence Mode II fracture characterization and, therefore, need to be properly addressed.

Delamination growth in composite laminates is essentially two-dimensional (2D), indicating a multidirectional spreading of interlaminar damage. However, the evaluation and prediction of delamination growth mainly relies on the quantification of one-dimensional (1D) growth using unidirectional specimens. In this study, the discrepancies and similarities between 1D and 2D delamination behaviours of composite laminates are investigated, both experimentally and numerically. The fracture toughness of mode II delamination, measured experimentally through 1D tests, is compared with the numerically fitted critical Energy Release Rate (ERR) in 2D delamination using Cohesive Zone Modelling (CZM) method. The fracture mechanisms involved in 1D and 2D delamination growth are investigated through fractography at the delamination interfaces. Although similar damage mechanisms are present in 1D and 2D tests, using the fracture toughness measured from 1D tests to predict 2D growth is proven to be insufficient due to distinct extrinsic toughening effects. Variations in local stress states significantly influence delamination growth, necessitating different cohesive constitutive models to accurately describe 1D and 2D delamination processes.

Given the long-term use of carbon fibre reinforced polymers (CFRP) in harsh environments, this study investigates the isolated and combined effects of temperature and moisture variations on mode I fatigue delamination propagation. Several levels of temperature and relative humidity were applied as preconditioning and as in-service during fatigue testing to evaluate their effects on the Paris curve. In addition, statistical analyses, including analysis of variance (ANOVA), semi-empirical interpolation modelling, and fractographic assessments, were conducted to provide a comprehensive understanding of the failure mechanisms. The results indicate that the moisture absorbed during hygrothermal preconditioning and the in-service temperature applied during fatigue test individually affect the Paris curve slope. These factors interact synergistically, significantly altering the fatigue crack growth rate. An empirical model capturing this interaction showed good agreement with experimental data, enabling reliable prediction of environmental degradation trends. Fractographic evidence supported the observed changes in fracture patterns, linking changes in fibre bridging formation, surface roughness, and energy dissipation to the observed shifts in fatigue behaviour.

Certification of composite structures remains a significant challenge in the aerospace sector. These materials exhibit various failure mechanisms under load, complicating the prediction of crack growth. Delamination is the most common and critical failure, typically triggered by combined tensile and in-plane shear loadings corresponding to Mode I and Mode II, respectively. Characterisation of Mode II remains particularly difficult due to the unstable crack propagation exhibited in many test configurations. This manuscript presents an experimental study of Mode II fatigue delamination at various R-ratios using the End-Loaded Split specimens, which enable stable in-plane shear-driven delamination. A multi-method approach utilising Digital Image Correlation (DIC), Acoustic Emissions, and post-mortem fractography analysis was adopted to provide a comprehensive description of how delamination behaves across varying R-ratios. The study was centred on the fracture process zone, measured via DIC, due to its significant impact on energy dissipation. Variations in the length of this zone throughout the fatigue life revealed an imbalance between the damage mechanisms affecting the growth of the true crack length and the effective crack length. This evolution of the fracture process zone was correlated with trends in acoustic energy dissipation and the morphology of the fracture surface. These findings provide new insights into Mode II fatigue delamination and enhance our understanding for the design of damage-tolerant structures.

The fracture process zone (FPZ) significantly influences the damage tolerance of adhesively bonded composite joints, governing crack-growth mechanisms and migration. Existing fracture characterization approaches generally evaluate pure-mode behavior independently and extend these results to mixed-mode conditions using a power-criterion, such as the Benzeggagh-Kenane (B–K) criterion. This process assumes that FPZ-dependent mode-mix behavior from a standard mixed-mode test is transferable to another complex loading condition. This assumption remains unchecked for toughened adhesive joints, where FPZ morphology varies with loading conditions. This study addresses this gap through experimental and numerical investigation using digital image correlation (DIC) and cohesive zone modeling (CZM). The pure mode I test displayed localized FPZ ahead of the crack tip, influenced by carrier bridging. Two different pure Mode II tests demonstrated that the apparent crack length method accurately accounts for the large FPZ ahead of the crack tip. The mixed-mode bending (MMB) test linked pure modes through the B–K criterion. The Crack-Lap Shear (CLS) specimens exhibited evolving FPZ and mode II-dominated fracture. The fracture toughness predicted by the B–K criterion deviated from the CLS tests as the loading became more mode II dominant. It was observed that the FPZ morphology during the CLS test differed significantly from that observed during the MMB test, through DIC and CZM. These results highlight that differences in FPZ affect the mixed-mode fracture toughness and demonstrate the limitations of applying a single empirical power-criterion. It underscores for FPZ-sensitive approaches to accurately predict the fracture resistance of toughened adhesive joints under evolving mixed-mode conditions.

This study presents a numerical analysis framework of multidirectional (MD) composite laminate under mode I fatigue loading, with calibration to account for fiber bridging. Both local and global methods for calculating the energy release rate are compared. Seven unidirectional (UD) and MD layups were tested, revealing differences in fatigue resistance based on fiber orientation and initial delamination length. Fiber bridging, where fibers stretch across separating plies, was found to enhance toughness. The results show that fiber bridging intensity varies with fiber orientation, with minimal stiffening for the 0°//0° interface and significant stiffening for 90°//90°, 0°//45°, and 0°//90° interfaces.

This study investigates the Mode-I fracture toughness of laminates with varying interface angles. A method for identifying crack tip location using grayscale characteristic parameters in DIC is proposed. The findings demonstrate that both initial and steady-state fracture toughness exhibit a bilinear relationship with interface angle. A cohesive constitutive model incorporating the interface angle was developed and integrated into a double cantilever beam finite element model, predicting delamination propagation behavior that was highly consistent with experimental results. Numerical analysis suggests that zigzag cracks may improve fracture toughness before steady-state toughness is achieved, with peak toughness correlating to the length of the zigzag cracks.

Where the trunk of a tree splits into co-dominant branches, wood fibres are highly interlocked. Such an arrangement of fibres has been shown to impart superior strength and toughness to this critical junction. Here, wavy patterns are 3D printed with a liquid crystal polymer (LCP) to evaluate the potential of wood-inspired localized adaptations to improve the robustness of junctions between orthotropic struts. The highly anisotropic, fibrillar microstructure of LCPs is harnessed by Fused Filament Fabrication, yielding Young's modulus and tensile strength reaching 30 GPa and 500 MPa respectively. However, unidirectional 3D-prints subjected to normal tensile stresses show weak interfaces, like in wood. To overcome this weakness, sinusoidal, helix and saw-tooth patterns are 3D-printed to create interlocking between layers. A trade-off is established between uniaxial tension and short-beam shear with increasing interlocking angle of the sinusoidal pattern. We find that the work associated with crack propagation in Mode I is increased three-fold compared to a unidirectional pattern, through extrinsic toughening. When applied to a more complex load case in a curved beam in four-point bending, helix-patterning at the junction zone increases the maximum load by 88 %. By locally controlling anisotropy via waviness, this method opens the possibility of improving toughness and transverse properties where the stress state is multi-axial without adding mass in future recyclable structural materials.

Multidirectional (MD) composite laminates are extensively employed in structural applications owing to their superior mechanical characteristics. Nevertheless, the evaluation of the fracture toughness of composite laminates primarily relies on tests using unidirectional (UD) specimens. This study evaluates the reliability of characterizing mode II delamination behaviour in MD laminates by using UD specimens. The quantification of delamination area through Digital Image Correlation (DIC) analysis is integrated with a physical Energy Release Rate (ERR) method to ascertain the fracture resistance, which is compared with the ERR derived via a modified J-integral method and the standardized compliance methods. Fractographic analysis reveals similar fracture mechanisms in specimens with identical interfaces. The physical ERR increases notably due to large-scale fibre bridging induced by fibre nesting at 0^∘//0^∘ interfaces. Conversely, in 0^∘//90^∘ interfaces, large-area matrix cracking enhances the intrinsic fracture resistance, excluding the extrinsic toughening provided by fibre bridging.

In this study, a novel experimental approach was devised to investigate shear dominant and combined opening-shear planar delamination behaviours in composite laminates subjected to quasi-static out-of-plane loading. The patterns of planar delamination growth were depicted through different inspection techniques, including digital image correlation (DIC), C-scan, and microscopic observation. The artificially embedded delamination propagated in the direction parallel to the fibre orientation of the layer above the mid interface, but migrated to an upper interface in the direction transverse to the directing ply. A continuous stiffening process was recognized with increasing delamination area. Furthermore, a numerical analysis based on virtual crack closure technique (VCCT) indicated that the local mode II was dominant for delamination growth, while the local mode III triggered delamination migration.

Continuous monitoring of spray velocity during the cold spray process would be desirable to support quality control, as spray velocity is the key process parameter determining the deposit quality. This study explores the feasibility of utilising Airborne Acoustic Emission (AAE) for real-time monitoring of spray velocity. Six spray tests were conducted, varying pressure and temperature to achieve different velocities. Optical means were used to measure velocity; while, the signal from the AAE was captured during deposition via a microphone. Features demonstrating a strong correlation with velocity were extracted from the acoustic signals. Both rule-based and machine learning models were employed to identify the moments where the nozzle was engaged with the substrate and diagnose the velocity. The results indicate that monitoring the spray velocity of the cold spray process using AAE is feasible.